The invention relates to an electron beam device such as a cathode ray tube having an electron gun, a focussing lens for focussing the electron beam produced by the electron gun on a desired spot on a surface and a glass envelope provided around a central axis of the electron gun.
More particularly, the invention relates to such a device in which the focussing lens is in the form of a helically coiled resistance layer provided on the inner surface of the glass envelope.
Such electron beam devices are well known in the art and are described in, for example, Vrijssen, U.S. Pat. No. 4,713,879, Spanjer et al U.S. Pat. No. 4,827,184 and Vrijssen et al U.S. Pat. No. 4,857,797.
While the performances of these devices have been proved to be satisfactory, it has been found that the reproducibility of these devices, particularly of the focussing lens, is not completely satisfactory. Thus it has been found that in a series of cathode ray tubes manufactured to the same specifications undesired variations occur in helically coiled resistance layers.
In addition it has been found that the resistance layers are insufficiently homogeneous and frequently exhibit undesirably high temperature coefficients of resistance.